Generally, when completing a subterranean well for the production of fluids, minerals, or gases from underground reservoirs, several types of tubulars are placed downhole as part of the drilling, exploration, and completions process. These tubulars can include casing, tubing, pipes, liners, and devices conveyed downhole by tubulars of various types. Each well is unique, so combinations of different tubulars may be lowered into a well for a multitude of purposes.
A subsurface or subterranean well transits one or more formations. The formation is a body of rock or strata that contains one or more compositions. The formation is treated as a continuous body. Hydrocarbon deposits may exist within the formation. Typically a wellbore is drilled from a surface location, placing a hole into a formation of interest. Completion equipment is placed downhole after drilling, including casing, tubing, and other downhole equipment as needed. Perforating the casing and the formation with a perforating gun is a well known method in the art for accessing hydrocarbon deposits within a formation from a wellbore.
Explosively perforating the formation using a shaped charge is a widely known method for completing an oil well. A shaped charge is a term of art for a device that when detonated generates a focused explosive output. This is achieved in part by the geometry of the explosive in conjunction with an adjacent liner. Generally, a shaped charge includes a metal case that contains an explosive material with a concave shape and has a thin metal liner on the inner surface of the explosive material. Many materials are used for the liner including brass, copper, tungsten, and lead. When the explosive detonates the liner metal is compressed into a super-heated, super pressurized jet that can penetrate metal, concrete, and rock.
A perforating gun has a gun body. The gun body typically is composed of metal and is cylindrical in shape. Within a typical gun tube is a charge holder or carrier tube, which is a tube that is designed to hold the actual shaped charges. The charge holder contains cutouts called charge holes where the shaped charges are placed.
A shaped charge is typically detonated by a booster or igniter. Shaped charges may be detonated by electrical igniters, pressure activated igniters, or detonating cord. One way to ignite several shaped charges is to connect a common detonating cord that is placed proximate to the igniter of each shaped charge. The detonating cord is comprised of material that explodes upon ignition. The energy of the exploding detonating cord can ignite shaped charges that are properly placed proximate to the detonating cord. Often a series of shaped charges may be daisy chained together using detonating cord.
Shaped charges are tested to ensure quality control as well as determine performance characteristics. A common test is to place a shaped charge on top of a plate and concrete cylinder. A steel jacket may surround the concrete cylinder. The test setup is typically located in a bunker for safety reasons. The shaped charge is then detonated remotely from a control station. The concrete cylinder is then opened up to determine the depth of the penetration as well as the deviation of the hole from the center of the cylinder. One problem with this method is that the concrete is always curing and is therefore not shelf stable for long periods of time. A further problem with concrete targets is that its properties (such as compressive strength and density) in general are difficult to control, resulting in inconsistent test data. Concrete is also too soft to gauge shaped charge performance in hard rock applications.
Natural rock targets are also commonly used for testing for improved accuracy of down hole charge performance. Berea sandstone is one of the most common natural rock targets. These rock targets are generally expensive. Moreover, availability of specific examples is sometimes limited. Rock targets also require complicated confinement designs to simulate the natural stresses in oil and gas producing formations.
Solid steel targets are used for targets. One problem with a steel target is that it is non-porous. An explosive jet passing through a porous medium versus a non-porous one may exhibit significant differences. This results in test data that is not always applicable to the field. Also, steel has a high compressive strength that makes it not suitable for simulating medium or soft formations.
Aluminum targets are also used to test shaped charges. Aluminum has the same problem as steel in that it is non-porous. Another problem with aluminum is that it may react with the materials in the high explosive jet. These reactions may result in disruption of the jet and erratic penetration patterns. Both of these problems result in inconsistent test data that does not always apply to field conditions.